For certain types of stations such as earth stations aboard aircraft (ESAA), for example, transmission towards a satellite (or ground-based stations) is limited by off-axis effective isotropic radiated power (EIRP) spectral density (ESD). That is, to prevent interference with adjacent satellites or ground-based stations, earth stations aboard aircraft (ESAAs) must comply with limitations on off-axis ESD. These limitations, which are typically stated as a mask specifying a maximum allowable ESD at a given angular offset along the geostationary orbit (GSO) arc, may either be coordinated with the operators of the adjacent satellites or mandated by a regulatory authority. Some phased-array antennas, such as those used in ESAA systems, exhibit isogain contour ellipticity and gain variation as a function of elevation. Due to the unique requirements associated with the airborne environment, the antennas used on board aircraft for communications may have complex antenna patterns that may vary as the aircraft moves throughout a given coverage area. As a consequence, the variations of antenna gain result in ESD limits that vary as a function of scan angle, skew angle, and frequency.
Adaptive, time-division duplexed multiple access (aTDMA) air interfaces may vary the coding, modulation, and symbol rate of transmissions from an ESAA to accommodate changes in the prevailing channel conditions, and other substantial capacity benefits on the link from the aircraft to the serving satellite (the “inroute”). However, since aTDMA transmissions can vary in frequency, power, and bandwidth on very short timescales, they introduce significant complexities in the monitoring of the ESAA's instantaneous ESD.
Furthermore, another method to meet the ESD limit is to select a minimum ESD limit over all operating conditions (i.e., scan angles, skew angles, and frequencies) used to transmit toward a particular target, and to calculate an antenna power input level using this minimum ESD limit. Providing power to the antenna using this minimum ESD thus ensures that the ESAA will meet the ESD limit for all scan/skew angle combinations as the antenna gain varies. However, this method has proven to be inflexible because it limits transmission to below the ESD limit for other scan/skew combinations when, for example, an ESAA antenna exhibiting a variable gain is capable of transmitting at the ESD limit across other scan/skew angles but is otherwise limited by the power output calculated in accordance with the minimum ESD.